The present invention relates to mass spectrometers, particularly to ion cyclotron resonance (ICR) mass spectrometers, and more particularly to a miniature ion trap mass spectrometer which combines an electron source, and the mass analyzer/detector assembly in a single device.
Ion formation, trapping, excitation and detection, in the environment of mass spectroscopy, are known techniques. Ion cyclotron resonance (ICR) is a known phenomenon and has been employed in the context of mass spectroscopy. Essentially, this mass spectrometer technique has involved the formation of ions and their confinement within a cell for excitation. Ion excitation may then be detected for spectral evaluation.
Various types of mass spectrometers and components thereof have been developed. For example, U.S. Pat. Nos. 4,588,888 issued May 13, 1986 and 4,668,864 issued May 26, 1987, each to S. Ghaderi et al., disclose a mass spectrometer including a cylindrical magnet enclosing an ICR cell in which sample ions are formed, trapped, excited and detected. U.S. Pat. No. 4,206,383 issued Jun. 3, 1980 to W. G. Anicich et al., discloses a miniature cyclotron resonance ion source using a small C-shaped permanent magnet.
Mass spectrometers which include a rectangular or cylindrical ICR cell disposed within a vacuum chamber, means to apply a static magnetic field in the region of the ICR cell, an electron gun to induce charges on gaseous samples, and means to perform Fourier analysis on the signals induced by the trapped ions in the trap electrodes, are disclosed in U.S. Pat. Nos., 5,233,190 issued Aug. 3, 1993 to F. H. Schlereth et al.; 4,990,856 issued Feb. 5, 1991 to W. A. Anderson et al.; 4,982,088 issued Jan. 1, 1991 to D. P. Weitekamp et al.; 4,931,640 issued Jun. 5, 1990 and 4,761,545 issued Aug. 2, 1988, each to A. G. Marshall et al.; 4,959,543 issued Sep. 25, 1990 to R. T. McIver, Jr. et al.; 4,581,533 issued Apr. 8, 1986 to D. P. Littlejohn et al.; 4,563,579 issued Jan. 7, 1986 to H. Kellerhals et al.; and 3,937,955 issued Feb. 10, 1976 to M. B. Comisarow et al. 5,155,357 issued Oct. 13, 1992 to H. F. Hemond, and 4,514,628 issued Apr. 30, 1985 to J. Friehart et al. disclose miniaturized non-ICR magnetic mass spectrometers.
In addition, U.S. Pat. No. 5,013,912 issued May 7, 1991 to S. Guan et al. discloses a method for reducing the dynamic range of Fourier transform ion cyclotron resonance (FT-ICR) signal generated by the stored wave for inverse Fourier transform (SWIFT) technique. U.S. Pat. No. 4,874,943 issued Oct. 17, 1989 to R. B. Spencer discloses gaseous ions trapped within an analyzer cell of an ICR mass spectrometer which are excited into resonance by a swept radio-frequency (RF) electric field having an envelope of trapezoidal shape. U.S. Pat. No. 3,742,212 issued Jun. 26, 1973 to R. T. McIver, Jr. discloses a method and apparatus for pulsed ICR spectroscopy in which a gas sample within an analyzer cell is ionized by means such as a pulse of an electron beam. U.S. Pat. No. 3,390,265 issued Jun. 25, 1968 to P. M. Llewellyn discloses a spectrometer which employs ICR and energy absorption in mass analysis.
Commercial mass spectrometer systems are available which utilize quadrupole radio frequency (RF) fields to mass analyze the specimen in continuous flow or to trap a sample and expel the ions into an ion detector by ramping the RF fields. Systems are also available which accelerate the ions from the source and pass them through dispersive electrostatic and magnetic elements. The ions are then detected in a separate region where they have been separated in space according to their mass and velocity. Resolution in these systems is achieved at the expense of either efficiency or size. In research laboratories Penning traps have been utilized in precision mass spectroscopy and have achieved a resolution of .DELTA.m/m=4.times.10.sup.-10 by detecting the ion cyclotron resonance frequency of single (or a few) particles in magnetic fields of up to 8.5 Tesla. See G. Gabrielse et al., Int. J. of Mass. Spec. and Ion Proc., 88 (1989) 319. These fields are produced by superconducting magnets. The ultra high vacuum needed to achieve this resolution is generated using cryogenic pumping. This low temperature environment is also extremely helpful in reducing electronic noise and hence making possible single ion detection. The large apparatus associated with these experiments is associated with the generation and maintenance of the cryogenic environment. Signal strength is improved and magnetic field homogeneity conditions are relaxed if the physical dimensions of the ion trap is minimized. Given that the ions are produced and detected inside the trap, the performance of the mass spectrometer is enhanced if the size is minimized. In addition, these prior systems utilize a separate ion source, mass analyzer and ion detector regions. Thus, there is a need in the art for a room temperature, portable, low power mass spectrometer with integrated electronics which has the capability for detection of environmental pollutants or illicit substances, for example. This need is satisfied by the present invention which provides a mini ion trap mass spectrometer based on the Penning ion trap principles using permanent magnets which will have an ultimate resolution of 10.sup.4 (at P=0.5.times.10.sup.-8 Torr), wherein power consumption is minimized by the use of the permanent magnets and a unique electron gun, and which combines a unique electron source and mass analyzer/detector assembly in a single unit, thereby drastically reducing to overall system size.